MicroRNAs are small RNA molecules of about 22 nucleotides. These microRNA molecules can control gene expression in a sequence specific manner in a wide variety of organisms.
In many organisms, RNA silencing mediated by double-stranded RNA (dsRNA), such as siRNA and microRNA, is part of an innate immune response against RNA viruses and transposable elements. Counter defense strategies to thwart the host response were found in, for example, plant viruses and the insect Flock House virus. These viruses express inhibitors, e.g., dsRNA-binding proteins, that interfere with the host cell RNA silencing machinery.
For example, microRNAs are reported to block translation after partially hybridizing to the non-coding 3′ region of mRNAs of target genes. The genes targeted by microRNAs largely remain to be characterized. However, there is growing evidence that microRNAs are implicated in various diseases and illnesses. For instance, drosophila microRNAs have been shown to target genes involved in apoptosis, and B-cell chronic lymphocytic leukemia has been linked to the deletion of two microRNAs.
However, to date, the existence of microRNA encoded by mammalian viruses have not been reported. Identifying mammalian virus microRNAs, and, if they exist, understanding their biological function would facilitate the development of new anti-viral drugs.
Therefore, there is a need to identify viral microRNAs, and for new materials and methods that can help elucidate the function of known and future virus microRNAs.
Due to the ability of microRNAs to induce RNA degradation or repress translation of mRNA which encode important proteins, there is also a need for novel molecules that inhibit DNA virus microRNA-induced cleavage or translation repression of target mRNAs.